Polytrimethylene ether glycol and its uses have been described in the art. It can be prepared by dehydration of 1,3-propanediol or by ring opening polymerization of oxetane.
U.S. Pat. No. 2,520,733, which is incorporated herein by reference, discloses polymers and copolymers of trimethylene glycol and a process for the preparation of these polymers from 1,3-propanediol in the presence of a dehydration catalyst such as iodine, inorganic acids (e.g. sulfuric acid) and organic acids. Polymers of molecular weight from about 100 to about 10,000 are mentioned.
U.S. Pat. No. 3,326,985, which is incorporated herein by reference, discloses a process for forming a polytrimethylene ether glycol having an average molecular weight of 1,200–1,400. First, polytrimethylene ether glycol which has an average molecular weight of about 900 is formed using hydriodic acid. This is followed by an after treatment which comprises vacuum stripping the polyglycol at a temperature in the range of 220–240° C. and at a pressure of 1–8 mm Hg in a current of nitrogen for from 1–6 hours. The polymer resulting from this process contains residues from the catalyst used.
U.S. Pat. No. 6,720,459, which is incorporated herein by reference, discloses a continuous process for preparation of polytrimethylene ether glycol from 1,3-propanediol using a polycondensation catalyst, preferably an acid catalyst.
U.S. patent application Publication No. 2002/0007043, which is incorporated herein by reference, describes a purification procedure for crude polytrimethylene ether glycol obtained from an acid catalyzed polymerization process comprising (1) a hydrolysis step to hydrolyze the acid esters formed during the acid catalyzed polymerization, (2) phase separation and water extraction steps to remove the soluble acid catalyst, generating an organic phase and a waste aqueous phase, (3) a base treatment of the organic phase to neutralize and precipitate the residual acid present, and (4) drying and filtration of the polymer to remove residual water and solids.
A method of purification of polyethers prepared using sulfuric acid catalysis is also disclosed in U.S. Patent Application Publication No. 2002/0049356, which is incorporated herein by reference. The process comprises heating the polyether with water for a period of 2 to 20 hours and at a temperature of 60 to 100° C. to substantially hydrolyze acid esters formed during polymerization, and then separating the polyether from the water. It may further include the step of washing polyether after hydrolysis with one or more additional water washes.
U.S. patent application Ser. No. 10/634,687, filed Aug. 5, 2003, (now US 2004-0225162 A1), which is incorporated herein by reference, discloses a process for improving the color of polytrimethylene ether glycol comprising contacting polytrimethylene ether glycol having color with adsorbent and separating the polytrimethylene ether glycol and adsorbent, wherein the polytrimethylene ether glycol, after contact with the adsorbent, has a molecular weight of about 250 to about 5000 and a APHA color of less than about 50.
It is clear from the prior art that when sulfuric acid is used as a catalyst to make polyether glycols from their corresponding diols, it is preferred to include a hydrolysis step because a substantial portion of the acid is converted to the ester, alkyl hydrogen sulfate. These ester groups act as emulsifying agents during the water washing process and thus cause the washing process to be difficult and time consuming and also make the acid removal incomplete. The hydrolysis step is also important in order to obtain polymer with the high dihydroxy functionality required to use the polymer as a reactive intermediate.
The purification processes disclosed in the prior art are effective in producing polytrimethylene ether glycol with high dihydroxy functionality. Often, however, it is desirable to produce short chain or low molecular weight polytrimethylene ether glycol from the polycondensation of 1,3-propanediol. As disclosed in U.S. Pat. No. 2,520,733, the trimethylene glycol polymers having molecular weights below about 200, are generally water-soluble. Polytrimethylene ether glycol with molecular weight below about 1,000 contains significant amounts of water-soluble oligomers. In addition to the solubility of oligomers in water, other factors such as the solubility of water in the low molecular polymer and interactions between polymer and water molecules may also exist. For this reason, the hydrolysis purification processes described above are very difficult for the case of low molecular weight polymers, because it is hard to achieve a distinct aqueous and organic phase separation. Also, the water washing steps utilized in the art are a substantial disadvantage, because the water washing not only removes the acid present but also removes the any water-soluble short polyether chains. Furthermore, in order to achieve high polymer yields, it is essential to recover the soluble fraction of the polymer from the water solutions, a process which is expensive and time consuming because it requires distillation of large amounts of water and creates high capital, maintenance, and operating costs.
It would be highly desirable if low molecular weight polytrimethylene ether glycol free of catalyst derived end groups, e.g. acid ester end groups derived from sulfuric acid, and other catalyst contamination could be prepared by acid catalyzed polymerization without the hydrolysis and/or water washing steps. The present invention is directed to these and other ends.